Packaging of organic light-emitting diodes using reactive polyurethane

ABSTRACT

The invention relates to an organic light-emitting diode containing polyurethane with free isocyanate groups for binding any undesirable atmospheric moisture that penetrates the packaging of the diode.

The invention relates to the packaging or organic, light-emitting diodesor displays, specifically to the protection of the diodes or displaysagainst atmospheric effects, especially oxygen and water or water vapor.

The Fabrication of Organic, Light-Emitting Diodes (OLEDs), the processesand materials used are described in detail in the literature. The readeris referred here to the “Philips Journal of Research”, 1998, Volume 51,No. 4, pages 467 to 477.

To guarantee a sufficient lifetime of the OLED a hermetic encapsulationis needed, since materials used for the OLED, especially cathodematerials with lower exit work, for example Calcium, but also polymers,react with oxygen and water. As a result of this degradation phenomenaoccur which reduce the lifetime of the OLED and thus prevent commercialuse of the corresponding modules.

A glass cap represents an effective protection from environmentalinfluences, said cap being able to be glued on for example by means of alight-cured epoxy adhesive. Selection of suitable adhesives allowsso-called 85/85 test criteria, meaning storage at a relative humidity of85% and of a temperature of 85° C. to be met over a period of time whichis sufficient for many applications, for example in mobile telephones.For other applications, for example in the automotive area however,higher demands are made.

Different measures are known for improving product lifetimes. U.S. Pat.No. 5,821,692 describes the use of perfluorinated fluids within the capcavity. U.S. Pat. No. 5,734,225 discloses the use of water-repellingsilicon layers in combination with yet further inorganic layers EP 0 884930 A1 discloses the use of water-absorbing polymers, for examplepolyimide, polyvinyl alcohol and polybutyral, likewise in combinationwith further inorganic layers. The use of inorganic materials, forexample zeolite-based getter tablets within the cap cavity, is knownfrom U.S. Pat. No. 5,882,761. All these materials react reversibly withwater, which means that the water can be given off again, especially ata higher temperature and can damage the OLED.

Using this as its starting point, the object of the invention is toimprove the protection of encapsulated components against effects fromthe atmosphere, especially oxygen and water or water vapor.

This object is achieved by the inventions specified in the independentclaims. Advantageous embodiments are produced by the dependent claims.

For protection against the effects of the atmosphere, especially oxygenand water vapor, an organic material is used which reacts irreversiblywith water. In principle hygroscopic materials, that is those whichreact with water from the air, are suitable for this purpose. Suitablematerials should in addition be inert in relation to the materials usedto fabricate the OLED. This also applies to the reaction products of thematerials with water.

The module to be protected is especially a package encapsulating one ormore organic components. It features a capsule to protect the componentsfrom environmental influences, especially from moisture. The capsule canbe present in the form of any given container for the component,especially a sealed container. To bind in humidity which penetrates intothe capsule or is already present in it an organic material which reactsirreversibly with water is disposed in the capsule.

From the plurality of possible materials available, those with freeIsocyanate groups have been shown to be particularly effective. Thesefree Isocyanate groups can react with the undesired water.

Free isocyanate groups can be made available at low cost through the useof hygroscopic, water-reactive polyurethane, which contains residualfree isocyanate groups. The polyurethanes can then be perceived to acertain degree as prepolymers which react irreversibly under the effector water, for example through cross-linking.

The material is preferably disposed in a cavity of the capsule andshould fill this cavity either completely or almost completely.

The capsule can for example be formed from a substrate and a cap intowhich a cavity is made.

If the capsule is to be transparent glass can be used as the materialfor the substrate and/or the cap. Alternatively caps made of metal orceramic can also be used.

The organic component is preferably an electro-luminescing component,especially a light-emitting diode.

The component can be arranged on the substrate and covered by the cap.

With a method for encapsulation of components, especially organiccomponents, material which reacts irreversibly with water is disposed ina capsule which contains at least one of the components. This materialpreferably features free isocyanate groups and is furthermore preferablypolyurethane.

The material can be applied locally by dosing, especially dispensing.

As an alternative or in addition the material can be applied to thesurface by screen printing.

Further significant advantages and features of the invention areproduced from the description of exemplary embodiments. The Figuresshow:

FIG. 1 a free isocyanate group;

FIG. 2 a module in the form of a package encapsulating an organiccomponent.

The isocyanate group shown in FIG. 1 is very reactive and reacts underrelatively mild reaction conditions and with compounds which carry acidsor active protons, for example water.

Plastics which are formed through polyaddition of multi-functionalisocyanates and compounds (alcohols) carrying at least two hydroxylgroups are referred to as polyurethane.

Suitable polyurethanes in accordance with the invention are those with arelevant residue of free isocyanate groups, that is those which areknown as moisture-curing single-component systems. These are used asadhesives and sealants in industry, for example in the field of glassgluing. They are produced from diisocyanates and dioles, in which caseit is necessary to ensure that a residue of free isocyanate groups isleft. These groups react irreversibly with water, in which casecarbamine acids are formed which in their turm form with hydrolysis ofcarbon dioxide amines These in their turn react with free isocyanategroups of other molecules forming substituted carbamides. Thus awater-initiated cross-linking between polymer chains occurs.

The production of oligomers and prepolymers based on polyurethane occursthrough the reaction of diisocyanates and dioles. Some of the mostwidely-used basic components are listed below. On the diisocyanate sidethese are for example toluoldiisocyanate, diphenyl methane diisocyanate,hexamethylene diisocyanate, xyloldiisocyanate and isophorondiisocyanate. On the polyole or polyhydroxy compounds side,hydroxyterminated polyethers, polyesters, polyolefines and glycolsshould be mentioned. The production of polyurethane and the reactions ofthe isocyanate group with water amines are described in the literature.

As stated, the polyurethanes mentioned are widely used as glues orsealants. For inventive use of the reactive polyurethane however notonly the adhesive or sealant effect is decisive but the ability of thefree isocyanate groups still present in these polymers to react withwater. The use of the reactive polyurethanes claimed for the OLEDrelates to the irreversible chemical reaction of the isocyanate groupswith water. The reactive polyurethane is thus used as an irreversibleorganic water getter. The reactive polyurethanes used can be filled withfillers known from the area of adhesives and sealants.

For processing reasons the polyurethanes which are especially suitableare those which have a paste-like consistency. They can then be appliedfor example by means of dispensing or screen printing. Advantageouslythe quantity of the dose is such that after the assembly of the jointsof a capsule, the dosed material fills out the volume of the cavity ofthe capsule to be filled completely or almost completely. The reactivepolyurehane and the adhesive used for gluing-on the cap of the capsulecan both be located on the same joint section, but also separately oneon each of the joint sections.

A decisive requirement for using the polyurethanes is their inertnesswith regard to the materials normally used for fabricating an OLED. Thisapplies especially to the metals which are as a rule used for thecathode with low exit work, for example calcium. Although groups areextremely reactive they are only react in the proposed application asintended with water which is penetrating but not with calcium or othermaterials used in the fabrication of the OLED. In addition, as a resultof the formation of carbon dioxide, no overpressure damaging the OLEDoccurs. This also applies to the OLED packaging, meaning of the gluingof the OLED with a glass cap. Surprisingly there is OLED compatibilityin the case of the reactive polyurethane.

FIG. 2 shows of a cross-section through a component 10. In this case anOLED 12 is arranged on a glass substrate 11. The OLED 12 is covered by aglass cap, the edge 14 of which is glued to the glass substrate 11.Glass cap 13 and glass substrate 11 form one capsule. The OLED 12features the following components: A transparent electrode 15, made ofITO (indium-tin-oxide) for example, an organic hole transport material16, for example made of a conductive polymer, an organicelectroluminescing material 17, for example a light-emitting polymer,and a metal electrode 18, which is made up of calcium 19 and silver 20for example. The organic electroluminescing material 17, which means theemitter (chromophore), serves in this case simultaneously as theelectron transport material. The two functions can however also beseparate, in which case the electron transport material is then arrangedbetween metal electrode and emitter. Above the component arranged on aglass substrate 11 in the form of the OLED 12 is the cavity of the cap13 filled with a material 21 which reacts irreversibly with water. Thismaterial 21 is preferably polyurethane with residual free isocyanategroups.

The fabrication of the component in the form of an organiclight-emitting diode is undertaken for example by spin coating ifpolymer solutions are processed or by vapor deposition if monomers areused. ITO (indium-tin-oxide) coated glasses are used as a substrate inwhich case the ITO can also be structured. ITO is transparent and isused as a anode because of its electrical properties. If necessary helplayers such as hole- and electron-conducting layers are used. Metalswith small exit work such as for example calcium are applied by vapordeposition as cathodes. The diode with the class cover is packaged andalso the material which reacts irreversibly with water in the form of awater-absorbing layer is applied in an inert atmosphere, which means inparticular in an atmosphere free of water and oxygen.

The details of the fabrication of light-emitting diode on the basis ofpolymers are as follows. In this case, to aid clarity, the fabricationof a diode without material which reacts irreversibly with water isinitially described.

Two parallel 2 mm-wide ITO strips at a spacing of 1 cm are created on anITO-coated glass substrate with 5×5 cm edge length and of a thickness of1.1 mm by means of photolithography. Exposed locations are not removedin an alkaline area. This protects the ITO. Revealed ITO is removed withconcentrated HBr at a temperature of 40° C. for two minutes. A 70 nmthick layer of PEDOT (polyethylene dioxothiophenone) is applied to theITO-structure glass wafer for example by means of spin coating from anaqueous solution. This layer is dried in a tempering process of 200° C.for five minutes. Subsequently the xylol emitter layer is applied at athickness of 100 nm, for example also by spin coating. This layer isdried at a reduced pressure of 10⁻⁶ mbar. Similarly at this pressure two2 mm-wide calcium strips are applied by vapor deposition as a cathode ata distance of 1 cm using a shadow mask. These metal strips are arrangedat right angles to the ITO-S structures located on the glass substrate.

The surfaces of the crossing anodes and cathode tracks, between whichthe polymers are located, represent the active surface of thelight-emitting diode. Silver strips to a thickness of 150 nm are appliedby vapor deposition to the calcium strips. No metal is deposited on theadhesive locations for example. The organic layers are removed manuallyfrom these locations. This can for example be done with a blade.

The polymer-free area can also be created as described in 03/03481 A2.Subsequently the four diodes obtained in this way are encapsulated witha glass cap. The method and devices described in 01/18886 A2 and01/18887 A1 can be used in particular to do this. The externaldimensions of the cap are 24×24 mm in the exemplary embodiment, theadhesive edge is 1 mm and the depth of the cavity 200 μm.

The parts to be joined are positioned in relation to each other in aninert, which means especially a water- and oxygen-free atmosphere, andfor example glued to each other with an organic adhesive.

If a voltage of for example is applied to the ITO electrode (anode) andthe Ca electrode (cathode) the encapsulated diode lights. The lifetimeobtained with such a diode when stored at a temperature of 85° C. and ofa relative humidity of 85% is for example 120 hours and serves as areference for the lifetimes stated below.

To increase the lifetime a material which reacts irreversibly with wateris used in the fabrication of the light-emitting diodes. Reactivepolyurethane is especially used for this purpose. The material is dosedby means of a dispenser centrally to the inside of the glass cap cavity.The quantity is selected so that when the glass cap is put on andpressed down the volume of the cavity is completely or almost completelyfilled, whereby the organic component is completely covered with awater-absorbing organic layer based on polyurethane. The glass cap isglued on after positioning by means of a light-cured epoxy adhesive. Theamount of glue needed for this is applied to the organic component bymeans of a dispenser.

When stored at a temperature of 85° C. and a relative humidity of 85°the lifetime of an OLED fabricated in this way is improved by the use ofthe reactive polyurethane by the factor of 3.

As an alternative or in addition the material which reacts irreversiblywith water can also be applied in the form of the reactive polyurethaneby means of a dispenser centrally to the organic component. When theglass cap is put on, the material is distributed so that the volume ofthe cavity is completely or almost completely filled up. The glass capis glued on after positioning by means of a light-cured epoxy adhesive.The adhesive bead needed for this is applied to the organic component bymeans of a dispenser. In this case too the lifetime of a OLED at atemperature of 85° C. and a relative humidity of 85% is improved by afactor of 3 by the use of reactive polyurethane.

1. Module with a capsule containing a component, especially an organiccomponent, characterized in that a material which reacts irreversiblywith water is disposed in the capsule to protect the component fromwater.
 2. Module in accordance with claim 1, characterized in that thematerial features free isocyanate groups.
 3. Module in accordance withclaim 2, characterized in that the material features polyurethanestructures.
 4. Module in accordance with claim 1, characterized in thatthe material is arranged in a cavity of the capsule.
 5. Module inaccordance with claim 4, characterized in that the material completelyor almost completely fills the cavity.
 6. Module in accordance withclaim 1, characterized in that the capsule features a substrate and acap.
 7. Module in accordance with claim 6, characterized in that the capfeatures a cavity.
 8. Module in accordance with claim 1, characterizedin that, the capsule is at least partly transparent.
 9. Module inaccordance with claim 1, characterized in that the component is anelectroluminescing component, especially a light-emitting diode. 10.Module in accordance with claim 1, characterized in that the componentis fabricated from organic semiconductors.
 11. Module in accordance withclaim 1, characterized in that the component is arranged on a substrate,especially a flexible substrate.
 12. Method for encapsulating acomponent, especially an organic component, in which a material whichreacts irreversibly with water is disposed in a capsule to protect thecomponent from water.
 13. Method according to claim 12, in which thematerial is applied locally by dosing, especially by dispensing. 14.Method according to claim 12, in which the material is applied to thesurface by screen printing.